There are a wide variety of electrophotographic imaging techniques. One such process, known as migration imaging, involves the arrangement of particles on a softenable medium. Typically, the medium, which is solid and impermeable at room temperature, is softened with heat or solvents to permit particle migration in an imagewise pattern.
As disclosed in R. W. Gundlach, "Xeroxprinting Master with Improved Contrast Potential," Xerox Disclosure Journal, Vol. 14, No. 4, July/August 1984, pages 205-06, migration imaging can be used to form a xeroxprinting master element. In this process, a monolayer of photosensitive particles are placed on the surface of a layer of polymeric material which is in contact with a conductive layer. After charging, the element is subjected to imagewise exposure which softens the polymeric material and causes migration of particles where such softening occurs (i.e. image areas). When the element is subsequently charged and exposed, the image areas (but not the non-image areas) can be charged, developed, and transferred to paper.
Another type of migration imaging technique, disclosed in U.S. Pat. Nos. 4,536,457 to Tam, 4,536,458 to Ng, and 4,883,731 to Tam et al., utilizes a solid migration imaging element having a substrate and a layer of softenable material with a layer of photosensitive marking material deposited at or near the surface of softenable layer. A latent image is formed by electrically charging the member and then exposing the element to an imagewise pattern of light to discharge selected portions of the marking material layer. The entire softenable layer is then made permeable by application of the marking material, heat or a solvent, or both. The portions of the marking material which retain a differential residual charge due to light exposure will then migrate into the softened layer by electrostatic force.
An imagewise pattern may also be formed with colorant particles in a solid imaging element by establishing a density differential (e.g., by particle agglomeration or coalescing) between image and non-image areas. Specifically colorant particles are uniformly dispersed and then selectively migrated so that they are dispersed to varying extents without changing the overall quantity of particles on the element.
Another migration imaging technique involves heat development, as described by R. M. Schaffert, Electrophotography, (Second Edition, Focal Press, 1980), pp. 44-47 and U.S. Pat. No. 3,254,997. In this procedure, an electrostatic image is transferred to a solid imaging element, having colloidal pigment particles dispersed in a heat-softenable resin film on a transparent conductive substrate. After softening the film with heat, the charged colloidal particles migrate to the oppositely charged image. As a result, image areas have an increased particle density, while the background areas are less dense.
Migration imaging can also utilize a solid, multilayered donor-acceptor imaging element having a uniform fracturable layer of marking particles, a marking particle release layer, a supporting carrier or sheet, and an adhesive-coated acceptor layer over the marking particle layer. By locally heating the element in an imagewise pattern, the heated marking particles are softened. This diminishes their attraction to the donor portion to a level below that of the attraction of particles in unheated areas. The acceptor layer may then be stripped from the element, removing the imaged pattern of marking particles from the release layer. Such systems cannot, however, achieve high resolution image reproduction, because any image area of the particulate layer must be cohesive enough to be carried with the peel-away layer, yet break cleanly at a border with a non-image area. Serifs, fine lines, dot images, and the like often have undesirably ragged edges with such processes. Such imaging techniques are disclosed, for example, in WO 88/04237 to Polaroid Corporation.
Although migration imaging can be achieved by exposure with various types of radiation, the use of near-infrared radiation, having a wavelength of 700 to 1,000 nm, would be particularly desirable. Such radiation can be produced with laser diodes which are relatively inexpensive and consume little energy. Effective use of near-infrared radiation in migration imaging, however, requires the presence of a near-infrared sensitizer which tends to absorb not only near-infrared radiation, but also visible radiation. This is detrimental, because visible absorptions remain in the resulting image. As a result, the final image has a corrupt color balance, when the sensitizer is incorporated in the marking particles of the migration imaging system, or a discolored background, when the sensitizer is included in the migration imaging element. These problems have made imaging with near-infrared radiation undesirable despite its economic benefits.